During a thunderstorm, the sky acts like a giant battery: charge builds up in the cloud, and a lightning bolt discharges it. That charge is invisible, but cosmic particles (muons, which constantly rain down on Earth from space) can sense it. As a muon flies through a charged cloud, it speeds up or slows down slightly, and these changes reveal the voltage inside the cloud.
The GRAPES-3 telescope in India recorded the muon flux during thunderstorms for ten years. It turned out that nearly six times more muons arrive from the eastern side of the sky than from the western side. The reason is not the clouds themselves but the fact that muons come in two varieties, positive and negative, and there are more positive ones in the flux. Earth's magnetic field bends positive particles more strongly, so more of them arrive from the east, and they are better at 'feeling' the charged top of a thundercloud.
The scientists tested this picture with computer simulations. If you remove the difference between positive and negative muons, the east–west asymmetry disappears. So it is precisely the charge imbalance that makes eastern storms more 'visible' to the telescope. This is the first time the muon charge has been directly linked to the direction of a thunderstorm.
Now muon telescopes can be used as sensitive detectors of electric fields in the atmosphere. A storm that is invisible to the eye still gives itself away through the particle flux.
Muons are born in the upper atmosphere when cosmic rays collide with air nuclei. In one second, about one muon passes through the palm of your hand.
In Stanislaw Lem's novel 'Solaris', the planet's ocean manipulates gravity to affect the station's orbit. Here, a thunderstorm controls the particle flux, a similar motif of invisible influence.